As described in the above-referenced '522 application, the ongoing development of wideband signal transport technologies, including coaxial cable, fiber optic and wireless (e.g., radio) systems, have resulted in a multiplicity of communication systems that serve a diversity of environments and users, such as ISM (Industrial, Scientific and Medical) customers. A particular advantage of wireless service is the fact that it is very flexible and not limited to serving only customers having access to existing or readily installable cable plants. Moreover, there are many environments, such as, but not limited to portable data terminal equipments (DTEs), where a digital wireless subsystem may be the only practical means of communication. To provide digital communication service, the wireless (radio) subsystem is interfaced with an existing digital network's infrastructure providing power and legacy wireline links (that may contain one or more repeaters) to an incumbent service provider site.
FIG. 1 diagrammatically illustrates such a radio as having an ISM-band compatible (e.g., spread spectrum) digital transceiver 10. The transceiver 10 includes a transmitter section 11, that is operative to perform spread spectrum modulation and up-conversion of baseband signals supplied from a baseband processor or digital data pump 15 (such as a T1 framer chip) coupled over a digital communication link 16 (e.g., a T1 link) to a telecommunication network 17.
The output of the transmitter section 11 is an FCC-conformal band RF signal (e.g., ISM 2.4–2.4385 GHz, or 5.725–5.850 GHz spread spectrum signal). This signal is applied to a transmit input port 21 of a diplexer 20, which has an antenna interface port 23 coupled to an associated radio antenna 25. A receiver port 22 of the diplexer 20 is coupled to a receiver section 12 of the transceiver, in which the spread RF signal received from the remote site radio is down-converted and demodulated to baseband for application to the digital data pump 15.
The transmit and receive ISM band frequencies interfaced by the diplexer 20 with the antenna 25 are prescribed by one of two complementary frequency plans (e.g., a transmit frequency fT=2.462 GHz and a receive frequency fR=2.422 GHz for use by the local site radio). These frequencies correspond to those of a narrowband transmit path filter 26 installed between transmit port 21 and antenna port 23, and a narrowband receive path filter 27 installed between antenna port 23 and receive port 22. The other (complementary) frequency plan is employed by a companion digital radio at a remote site (e.g., having a transmit frequency fT=2.422 GHz and a receive frequency fR=2.462 Ghz).
To facilitate selection of either frequency plan, the radio transceiver-diplexer arrangement may be configured as disclosed in the U.S. patent to P. Nelson et al, U.S. Pat. No. 6,178,312, issued Jan. 23, 2001, entitled: “Mechanism for Automatically Tuning Transceiver Frequency Synthesizer to Frequency of Transmit/Receiver Filter” (hereinafter referred to as the '312 patent), assigned to the assignee of the present application and the disclosure of which is incorporated herein. In accordance with this patented scheme, the frequency plan (transmit/receive frequency pair) of the radio is defined by selectively coupling the appropriate one of the two diplexer ports to the transmit port of the transceiver and the other diplexer port to the receive port of the transceiver. (At the far end or remote site, the diplexer-to-transceiver port connections are reversed.)
Because the environment in which such a digital radio is expected to be used may not provide ready access to alternative communication services, an auxiliary or redundant transceiver, to be substituted or switched in place of the main or principal radio by an associated controller, in the event of an apparent failure of the principal radio, may be employed. However, the ability to switch in a back-up radio does not resolve whether or not there is indeed an operational problem with the main radio.
For example, in the event of an apparent failure in initiating or conducting communications between the local radio and a remote site, it would be desirable to know if the problem lies with the local equipment, or resides in the remote site. This is particularly true where the radio is located at a relatively inaccessible location, where maintenance services may not be not readily available.
Fortunately, this problem is readily resolved by the reduced complexity and cost ‘localized’ RF loopback test circuit disclosed in the '522 application, which is configured to be coupled to the antenna port of the radio's diplexer, and is operative to determine whether the radio is functioning properly. No communication with a remote site need be attempted. As shown in FIG. 2, this RF loopback test circuit includes a frequency generator (such as a crystal oscillator) 30 which is operative to generate an auxiliary frequency fA, which corresponds to the sum or difference between the transmit and receive frequencies employed by the radio 10. As a non-limiting example, for respective transmit (fT=2.462 GHz) and receive (fR=2.422 GHz) ISM band frequencies of the transceiver of FIG. 1, auxiliary frequency generator 30 outputs an auxiliary frequency fA=(2.462–2.422) GHz=40 MHz.
The output of the frequency generator 30 is (resistor-coupled) to a Schottky diode mixer 42, which is further coupled through an attenuator network 44 to an input/output port 46. Being coupled to each of frequency generator 30 and the input/output port 46, the Schottky diode mixer 42 is operative to produce respective output frequencies representative of the sum and difference of the (40 MHz) frequency output of generator 30 and the frequency of whatever signal is coupled to input/output port 46. Thus, by coupling the input/output port 46 to the diplexer's antenna port 23, then as long as the radio's transceiver section 11 is transmitting at fT=2.462 GHz, mixer 42 will produce a sum frequency fS (2.462+0.040=2.502 GHz), and a difference frequency fD (2.462−0.040=2.422 GHz, which corresponds to the receive frequency fR).
Each of these frequencies is looped back to the narrowband filters 26 and 27 of the diplexer 20. Since only the (difference) frequency fD=2.422 GHz is associated with either filter (the narrowband receive path filter 27), the diplexer's receive port 23 will output the (difference) frequency fD=2.422 GHz for application to the receiver section 12. The summation frequency fS=2.502 GHz, on the other hand, is blocked by both narrowband filters 26 and 27.
As long as the RF loopback test circuit 30 is operating correctly, then if the radio's receiver section 12 providing an indication that it is receiving sufficient signal level, it can be inferred that each of the transmitter and receiver sections of the digital radio is operating properly, and any failure of the radio to receive from a remote site can be attributed to a problem at the remote site, or a local problem with the feedline, connectors, or the antenna itself. On the other hand, if the radio's receiver produces no output, it can be inferred that there is a problem with the local radio.